1. Field of the Invention
The present invention generally relates to a thermistor (thermally sensitive resistor) monitor system, and particularly, it relates to a monitor system a light emitting diode (hereafter "LED") for monitoring a thermistor network composed of a plurality of thermistors of a kind, such as a positive temperature coefficient thermistor (hereafter "PTC"), a critical temperature resistor (hereafter "CTR") or a negative temperature coefficient thermister (hereafter "NTC"), to display an operating state of the thermistor network, indicating, if any, a thermistor having a significantly changed resistance relative to a normal state in an identifiable manner.
2. Description of Relevant Art
The thermistor is a collective nomenclature of electronic components each respectively comprising a semiconductor extremely sensitive to its temperature so that its electric resistance greatly changes in dependence on the temperature relative to a state in a temperature range under a normal working condition, and has a non-linear temperature coefficient that is positive in PTC or negative in NTC or suddenly drops in CTR at a critical temperature.
The thermistor has a wide range of applications, where it is frequently employed in the form of a network to be monitored with one or more LEDs. Such a thermistor network comprises a plurality of identical thermistors with or without interconnections therebetween.
For example, a PTC network is employed for protection of loads in an electrical system, and occasionally some PTCs conduct large currents due such as to faulty loads that may be short-circuited, having their temperatures increased, entering their apparent open states to interrupt current conduction therethrough. Such a PTC network needs monitoring in a manner identifiable of a respective PTC automatically operated to be apparently open.
FIG. 1 shows a conventional PTC monitor system in a simplified form, and FIG. 2, another conventional PTC monitor system.
The conventional system of FIG. 1 comprises a pair of monitor circuits 2a+3a, 2b+3b each comprised of an LED 2a or 2b and a resistor 3a or 3b connected in series. The monitor circuits 2a+3a, 2b+3b are each connected across a corresponding one of two PTCs 1a, 1b that are installed in a pair of electric circuits connected in parallel between a power supply and a ground. The electric circuits are each constituted with an electric load 4a or 4b, and one PTC 1a or 1b is connected thereto in series.
If the PTC 1a or 1b is operated with e.g. a short-circuit in associated load, a corresponding one 2a or 2b of the two LEDs has a changed brightness with a varied current conducted therethrough, thereby indicating an operating state of the PTC 1a or 1b.
The conventional system of FIG. 2 also monitors two PTCs 1a, 1b that are installed in a pair of electric circuits connected in parallel between a power supply and a ground, which electric circuits are each constituted with an electric load 4a or 4b. This system however comprises a single monitor circuit comprised of an LED 2 and a resistor 3 connected thereto in series. The monitor circuit is connected at its positive pole end to a power supply end of each PTC 1a, 1b and at its negative pole end via a forward diode 5a, 5b to a load end of the PTC 1a, 1b
If either PTC 1a or 1b is operated, the LED 2 has a changed brightness, indicating an operating state of a combination of the PTCs 1a and 1b.
In the former system, two PTCs are individually monitored by two LEDs, which means any increase in number of PTCs requires an identical increase in number of LEDs, causing an undesirable increase in number of circuit components accompanying a dear cost.
In the latter system, two PCTs are monitored with a common single LED failing to identify which PTC is operated to be apparently open.
Similar circumstances appear with respect to other thermistors.